Fault indicator for plural connected devices



NlLS-OLOF JOHANNESSON FAULT INDICATOR FOR PLURAL CONNECTED DEVICES Filed Feb. 4, 1953 7 Sheets-Sheet l Fig.7

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FAULT INDICATOR FOR PLURAL CONNECTED DEVICES Filed Feb. 4, 1953 7 Sheets-Sheet 5 72 22 32 L 2 13 I 2. I I 3 3 3 I I l I i l i i l 7,0 5; 2P 5 3p 5 910 5g L L L Lg o 0, D :1: p :11: N L :3

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FAULT INDICATOR FOR PLURAL CONNECTED DEVICES Filed Feb. 4, 1953 7 Sheets-Sheet 6 INVENTOR NlL L Oi-DQNNRSSON r" 4&1

ATTOR EYS June 19, 1956 NlLS-OLOF JOHANNESSON 2,751,578

FAULT INDICATOR FOR PLURAL CONNECTED DEVICES Filed Feb. 4, 1953 7 Sheets-Sheet 7 IN ENTOR NIL L F HBNNE N r" w -IL ATT R EY FAULT INDICATOR FOR PLURAL CONNECTED DEVICES Nils-Olaf J ohannesson,

Telefonaktiebolaget L a company of Sweden Application February 4, 1953, Serial No. 335,132 1 Claim. (Cl. 340-453) Hagersten, Sweden, assignor to M Ericsson, Stockholm, Sweden,

This invention relates to common alarm giving means for signalling faults within a group of electrical devices, particularly equipments for long distance telephony.

In modern installations for long distance telephony it is desirable to work with a minimum of periodical checkings and maintenance at the same time as faults within the difierent equipment must be rapidly detected and repaired. Electromechanical relays have previously been employed for supervision in such installations and these relays have been connected to anode circuits of tubes important for the operation, to carrier frequency voltage sources etc. A complete supervision in this manner gives, however, rise to several troubles, partly owing to the great number of relays needed, and partly owning to the fact that high frequency energy is consumed by relays requiring a comparatively great amount of energy. The purpose of this invention is to construct devices giving the possibilities of a more complete supervision with less complicated devices and a smaller power consumption at the controlled place.

The invention is based upon the principle, that every supervised device acts upon the alternating current impedance of a control apparatus, which is connected to a control Wire. The control wire is in its turn connected to an indicating equipment, which can measure and signal a change of the impedance caused by abnormal operation of the device, to which the control apparatus is connected. The control apparatus is arranged in such a-manner, that such abnormal operation causes a change of impedance many times greater than that caused by normal variations in the operating conditions. A great number of control means from different supervised devices may be connected to one and the same control Wire, whereby the alarm equipment is simplified. In order to determine from which of the controlled devices connected to one and the same control wire that the alarm comes, a special measuring switch is included in the apparatus according to the invention, which switch may be operated manually or automatically.

By making the indicating equipment sensitive to changes in the alternating current impedance two advantages will be obtained: A galvanic contact between the control wire and the controlled devices is thus unnecessary, and the measurements can be performed by means of small voltages and powers, which later may be easily amplified.

The apparatus according to the invention for indicating abnormal operation of one or more devices within a group of such devices comprises an indicating equipment responsive to changes in the impedance between control wires, to which control means individual for each device within said group are connected, whereby during normal operation the impedance of each of said control means is arranged to vary continuously in response to the current or voltage condition Within the corresponding device. Said apparatus is mainly characterized by the fact that the impedance alteration of each control means is extremely great when the current or voltage condition within the corresponding device exceeds the limits. fixed for its maximum deviations during normal operation, and that the impedance alteration of any control means upon abnormal operation of the corresponding device is appreciably greater than the composite impedance variation resulting from other control means assigned to devices, which are operating within said limits.

As all the control means are connected in exactly the same manner to the control Wires and the indicating equipment, the change of the impedance caused by abnormal operation ought to manifest itself either as shortcircuit or as interruption, the indicating equipment being suitably arranged for the respective case.

In the first mentioned case all control devices are arranged in shunt to the control wires, whereby abnormal operation of a certain device gives rise to a very low impedance within the corresponding control device.

In the second case all control devices are connected in series with the control wires, whereby abnormal operation of a certain device gives rise to very high impedance within the corresponding control device.

A combination of both above-mentioned cases is also possible, whereby two control wires have one of their ends connected to a constant impedance and the other ends connected to the indicating equipment and the con-- trol devices are connected in said control wires. vices will then give a series and in parallel with The parallel connected control deabnormal operation of the respective devices.

Further characteristic features the accompanying drawing, wherein a number of embodiments and details of the apparatus according to the invention are shown.

Fig. 1 in the drawing shows a block diagram of an apparatus according to the invention. Fig. 2 shows a block diagram of another embodiment of said apparatus.

Fig. 3 shows a block diagram of a combination of the devices shown in Figs. 1 and 2.

Fig. 4 illustrates a B-H-curve of an iron core. Fig. 5 illustrates the inductance of a coil as a of its number of ampere turns.

Figs. 6 to 8 show diagrammatically the windingsof some coils.

Fig. 9 shows the current-voltage characteristic'ot .a

crystal diode.

Fig. 10 shows the difierential resistance of a'crystal diode as a function of the bias of said diode.

Figs. 11 to 17 apparatus according to the invention.

Figs. 18 and 19 show other embodiments of control means belonging to the apparatus according to the invention.

Figs. 20 to 25 show difierent embodiments 'of the in dicating means of an apparatus according to the'invention.

In Fig. 1 the main principles of an apparatus accord ing to the invention are illustrated. A number of devices App are to be supervised in common" by means of an individual control device ferent control devices K are with the control Wires m and n, structed in accordance with the priate subsequent figures of the In Fig. 2 there is shown and each may be conshowing in any appro drawings.

in series with the control wires m Patented June 19, 1956.

very low impedance and the series connected control devices a very high impedance during,

of the invention will appear from the following description in connection with function illustrate different embodiments of the 1 control means associated with devices supervised by the another embodiment wherein the devices App and their control devices Kare connected and n, which latter in case 'are connected together at the far end as seen from the'i'nd'icating equipment l'ndi Fig. 3 shows diagrammatically a combination of the a r f mer ers two methods represented in Figs. Land 2, fonthe COHHQC'? ti'onrof th'elsupervised ldevices to the, control'wires m and n; In this case the control wires are. terminated By a: constant impedance Z at the far end as. seen from the indicating equipment 171d. Between the two endsof the controlwirescontrol' means K ass'ociated'with the. dilferent device's App" are connected alternatively in parallel andinseries.

indicated above abnormal operation. ofany ofthe deVic'esKpp will give rise to a. very low impedance in parallel connected; controlimean's K. pedancein series connectedcontrol means. The abnormal operation will evidently correspondto short-circuit and interruption, respectively;

control means K used in the general'systems of Figs: 1"to 3, inclusivemay'bec'onstructedin differentm'anners; A possibilityfor supervising a' current is thus to use an indirectly heated thermosensitive' resistance (thermistor): Such a thermistor'consists ofa small, very temperature" sensitive, by"a heater spiral. Theicurrent, which is'to be: supervised, is caused to'flow through'the' heater: When the value of the current" differs 'from the' normal value the, resistance of said" element is changed either toa very 'high' or'toa very'low value; dependent'upon if the current has decreased or'increas'ed; anduponthesign' of the tempera ture coefiicient ofjthe'thermistor:

'l?he" .co'ntrol means K may also"consist"of'magn'etical'ly" saturated iren= cores; Magnetic materials are now availa'liIewhieh'have avery'steep 'pa'ssa'ge'from' a'magneticall'y' saturated toa magnetically'non saturated'conditioni The curve etlhe'indirction B as" a function of the" field strength-H forsuch a material isshown in Fig; 4. If a coil -is wound on such an iron core, the inductance of the coil as a function of the current I will beapproxi mately as indicated in Fig. 5, whereinthe inductance L is shownas afi inction' of the mag-netieal-field strength H-IN. It is clear from- Fig; 5 that E=Lr if [l[ ln while L=Lz=for Il[ Io, L1 being much greater than L2,. In is a constant dependent upon the core ancl'ihe' number ofiwinding'turns-N The indicated properties of such a coil maybe utilized for control devices-'K in an' apparatus according-tot the inventiona For this purpose acore having-the mentioned properties is provided with shownginEig. 6. Thewin'ding 1;.-2 is via-the control-wire connected to the indicating equipment Ind (Fig: l-3-)', while? thewinding. 34 is passed by a direct-current Isa which is to be supervised. When in'g:current impedance? Z12: of winding l-Z- will he very small, and if lIa4[Y Ib',:.Z1z'willbe=very high. Dependent upon 1 if ll| lnion [It 10 1' corresponds to abnormal operatiom the difierent'controhdevicesare thus connected in series (Fig. 2) or in. shunt. (Fig.2

Tihefcircuit shown; in: Fig.1 6 has; some? disadvantages:- A certain crosstalk may thus be obtained between: the difirent controlled-devices; because: of; alternatings voltages heingjransfornred. from" winding 1i-2 itO winding 3-4, and the direct current 134 has moreover to be ap' plied; toiwinding alternating current resistance. Thesedisadvantages may be eliminated by 'm'eans of a' circuit according to "Fig. 7 ine whichcireuit two iron cores areutili'zed'i- The" twoterrnihal's l and '2" are connectedvia the control wire to' the-indicatingequipment Ind and theitwo't terminals 3 and" 4"are' connected to the. supervised direct current in theisame manner as described above. The two coils. are. identicallyequal, but the windings are connected. in such a manner, thatanxal'ternating current through 1' 2 will give-alternatingvolt'ages across winding S- I, whi'chare. ofoppositedirections and thusneutralize eachotl'ier The resistive element, which is heated' lI34[ Iu, the alternat l): with: thecontrol 3-4 from a circuit having a very high 7 function of the device shown in Fig. 7 is for the rest the 7 same asthat'ofthe deviceshown inFig; 6';

By adding other windings 5, 6 as shown in Fig. 8 through which windings a constant direct current is passed counteracting the direct current 134 it is possible to get an indication when the current las is. somewhat greater than its normal value as well as when'it is somewhat less than said. normal value. The limits .WilllwfhfilldPGlldi uporntlie magnetical properties of the cores, the number of turns of the windings and. the value of the currentaIss. Abnormal operation will here evidently cause. a. low iin' pedance between the output terminals I 7 Another manner of arranging a control device K i s to utilize-the non-linear properties. oflre'ctifiers The-currentvoltage characteristic of e. g. a. crystaldiode is illustrated in Fig. 9. The difi'erential resistance R for small alternating voltages varies thereby with different biases U1. in the manner. shown in Fig, 10.

If the. voltage Ur. varies from some volts minust'o'some volts plus, said resistance R will be changed from'a-liigh" limit value R1 to a low limit value R2. is usually very high; often about 10 or"10 andachang'e of. only some tenths of' 21 volt arouncr'the value Ui-=0'- will give such a great change of'R, thatifrnay be'con= sid'ered eitheras' a short-circuit'or. an interruption ofthe true for other" The above statements are also" kinds of rectifiers, as rectifier tubes-with"hot'cathode'; cop?" peroxide and selenium rectifiers.

The mentioned properties 'of rectifiers may be-utilized" forthe construction of a control deviceK; which issensi tive for changes'of a direct voltage. The abnormal op eratiorr maythereby correspond to an interruption; R- Rr, or short-circuit; K- Rz. Thelatter" case will; however; give a' simpler constniction, for'whi'ch'rea'son only circuits in accordance-herewithwill be descril ed in thefollowing.

The control devices K are with the control wire to the indicating device;"which' de vice therefore onlygives alarm for short circuit-:

The direct voltage-U; whichis" to lie-supervised in a certain-control device; is compareiwi-th a fik bias voltage Em; The difiereuce volt-age (Erie-U)- is caused to actuate a rectifier, which as seen from the-alternating current shown in Figpl'l hasa rather high value. In-thisaca'se U is normally greater than E0; the. rectifier:L-is--non-con ducting and theimpedance'Zm between-the: terminals 1 andi connected to the. control to -R2. If the vol'tageU' across: the terminals13:and"4 con nected: to the supervisedidevice decreases withf a=certain1 amount below its. normal value, so that UEn', the recti fier L becomesrconductin'g; the-input: imPGdBEIICQ'Zir-r will? be low and the indicating equipment Ind will' give-alarm;

If, instead, alarm is wanted; when the volt'age U of the supervised device has increased with a.- certain amount above its nominal value; the polarity of the is reversed and the volta'ge Eo= is-increased to a cor-respo'nd' ing' degree. If it iswanted to-give alarm forboth posi-- tive and' negative biases and rectifiers with reversed polaritiesin-compari son with eaeh" other, will be'util'ized' 'according*to: Fig: 1 2.

In thecircuits shown in 'Fi'gs. 1 1 andS: l2'thec0ndnser Cr may be omitted if the alternating currentimpedance OfRi is low enough.

TheresistorsiRsand' R21; R22; respectively;, are deter= minedby two contradictory conditions; thus they must'not' have too small. a' resistance so tlia't'the control wire is' loaded'too much, and further they must not have tOOhig'h. a resistance, so that the direct; current through'the recti fiers is limited to too small 'a value. These difiicultiesmay be avoided by usingcoils which havealow directcurrenp The ratio'Ri/Rz thus connected" in' paralleh Wires is high and. equal deviations t'wo branches-with different impedance and a high alternating current impedance. Fig. 13 shows such a coil D inserted instead of the resistor R2 in Fig. 11.

Another method is to insert an additional rectifier as is shown in the circuit of Fig. 14. During normal operation U is greater than Eu and the two rectifiers L and L2 are non-conducting, i. e. Z111 is high. When U- Eo, L1 and L2 will begin to conduct, i. e. Z111 will be low and alarm is obtained.

The circuit diagrams in Figs. 13 and 14 relate to devices giving alarm for too low voltages. In a similar manner alarm may be obtained for too high voltages by reversing the polarity of the rectifiers in Figs. 11 and 12 and increasing the bias E0 to the upper alarm limit.

In order to obtain alarm for both too high and too low voltages the resistors R21, R22 in Fig. 12 may also be replaced by coils or rectifiers. Afurther way is to use only one bias E0, but inserting a voltage divider for the voltage U, which is to be supervised. Figs. 15 to 17 show this principle whereby the bias E0 is applied in series with the resistor R2, the coil D and the rectifiers L11, L21, respectively.

In these cases the voltage divider R11, R12 and the bias B0 are adjusted so that in normal cases R12 R.1+R1.

Alarm will then be obtained, if s o 12 i..e. alarm will be obtained for both too high and too low voltages.

In the circuit diagrams according to Figs. 15 to 17 the condensers C11, C12, C2, C3, C31 and C 32 ought to have a low impedance for the measuring alternating voltage. The condensers C11 and C12 may be omitted, if the alternating current impedances of R11 and R12 are low enough.

For a certain type of rectifiers the bias difierence AU between practically interrupter and practically shortcircuiter impedance will be constant. By having the normal value U0 of the voltage U different in difierent devices and the value of E0 in each individual case equal to (Uo+AU) or (Uo-AU), it is possible to obtain alarm for difierent percentages of deviations of the devices. The cathode voltage of a tube need e. g. only be controlled with regard to its lower limit with a fairly great tolerance, while on the other hand a direct voltage, detected from a pilot frequency in a carrier fiequency system has to be supervised with considerably narrower limits in both directions.

-In the figures one of the control wires has been shown grounded, which is advantageous in practical constructions. The condensers C2, C3, (C31, C22) serve to galvanically separate the two points from ground between which the direct voltage is to be supervised.

- It is clear from the foregoing that one and the same indicating device may be used for supervising direct voltages with difierent potentials in relation to ground, negative and/or positive deviations, and with difierent tolerances for difierent devices.

If there is a group of direct voltages with e. g. one of their poles at a common potential, the condenser C2 in Figs. 11 to 17 ought of course to be common for this group. If further the voltage E0 is equal for the whole group the condensers C3 respectively C31, C32 may be omitted and the bias E0 may be applied in common through a resistor, coil or rectifier. If in this case one pole of the direct voltages is grounded, the condensers C2 may of course be omitted.

Fig. 18 shows an example of such a device, wherein one pole is grounded, which device gives alarm for too low voltages, and wherein the bias E0 is applied via a rectifier. In this case alarm will evidently be obtained, if any voltage U is less than E0. The terminals 1-2 are connected to the control wire in the same manner as described above.

The back resistances of the rectifiers are however in fact not infinitely great and their forward resistance is not infinitely small. This will limit the number of control devices, which may be connected in shunt with the same pilot wire. The number of devices can be increased considerably, if the circuit shown in Fig. 18 is utilized and the rectifiers are arranged in groups as is shown in Fig. 19.

In normal cases the voltages of the points 11, 12 etc. are always greater than E0 and all the rectifiers are nonconducting, i. e. only a very small current is flowing. The impedance Z111 will then be equal to the back resistance of L, shunted by an impedance, which cannot be less than the parallel connected back resistances of the rectifiers L1, L2 Lq. In this manner the number of parallelconnected rectifiers is limited to (q-I-l) in the normal case instead of q.p as it would otherwise have been. If now the voltage is supposed to decrease below its allowed value E0 at some point, e. g. at point 33, a rather great current will pass through the rectifier L33, L3 and L, causing their alternating current impedance to be low so that indication will be obtained.

After the indication of a fault the question of where this fault is located may be examined in a great many manners. At first it may be proved e. g. in which group the fault is located by disconnecting respective switches S1, S2 a separate instrument be examined at which point the direct voltage is too low.

The number of groups and their further division must of course be adapted to the wanted number of measuring points and the characteristics of the rectifiers and the division into groups becomes thus more complete for a larger installation.

Fig. 19 shows a circuit intended to give alarm for too low a voltage. It is possible to obtain alarm for too high a voltage in the same manner, if the rectifiers are reversed. The rectifier L may also be replaced by a coil which has a low direct current impedance and a high alternating current impedance.

The devices which are supervised by the control means K constructed according to any of the embodiments described above, may be more or less sensitive to the alternating current, which is used for the impedance measurement. The interferences caused thereby may however be easily eliminatedv by decoupling by means of resistors, condensers and inductances in the control device, the pair of terminals, which is connected to the supervised device. Such a decoupling will also prevent crosstalk between the supervised devices.

The indicating equipment of .the apparatus according to the invention may be constructed in different manners;

Some possible .embodiments of such indicating equipments will be described in the following.

In the indicating equipment according to Fig. 20 the designation 0 indicates an oscillator, which continuously delivers an alternating voltage, and L indicates alarm means giving fault indications. The oscillator O is connected to the alarm means L through an attenuator D. The control wire is connected to the pair of terminals 1-2 of the attenuator D. The attenuator is constructed in such a manner that the great change in the impedance of the control wire arising as a result of a fault gives a very great change in the trasmission properties of the attenuator. from the pair of terminals 3-4 to the pair of terminals 56.

The device according to Fig. 20 may be constructed in two difierent manners, so that a fault within a supervised device gives either a low or a high attenuation in the attenuator D. In the first case the alarm means L has to give alarm, when an alternating voltage from the oscillator O is applied to the pair of terminals 56, and in Sq shown in Fig. 19, and it may then by means of the-latter case 'alarrnimustbe given, when the: alternating:

voltagedisapp'earsi fronr said pair. of. terminals. The last mentionedarrangement gives better? control, becausein this casealsorafault intheoscilla-torO: or the alarm-devicesLcauses alarm.

.ThepthattenuatorD changes its; transmission properties only at thefrequency f whenthe'controlwire connected to'Dnchan'ges its impedariceas the result of a fault. ,The attenuation of. Dg is moderate, for other: frequencies; The: frequencies fr, f-z fist of" respective attenuators are; alldifferent;

The oscillator may be. constructed so as'to simultaneously transmit all the said frequencies. If the attenuators normally have a low attenuation at the respective frequencies f1 fit the alarm device L hasv to be con:- structedso that disappearance of an alternating voltage of. a certain frequency will: cause alarm. ranged e. g. by the alarm device beingprovided with selective means with separate alarm transmitters for each frequencyisfn. Another: possibility is-to provide the alarm means L-with only one alarm transmitter having variable tuning. This tuning isvaried periodically asa functionof time', .so-=thatoneby one of the transmitted signals. of the frequencies. f1 fr: will. be received. Each signal. will. thenv be'received for. such. a long time-that possible disappearance can provoke alarm givingr Instead of having periodically varying timingof: the alarmmeans'Lthe oscillator 0 may be constructed to transmit only one frequency, which. is madeto obtain. one by one of thevaluesfr fN. periodically. In this casethealarm means isconstructedtohave equal'sensitivity for all thefrequencies f1 in. When a fault causes a high attenuation e. g. in Dp signals .to the alarm meansv L will lack, while the oscillator transmits the frequency f and this for atime sufliciently long for alarm.- If onv the other. hand the attenuators. have normally a high attenuation. at their. respective frequencies f1 fit alesscomplicated deviceis obtained. In such a case the alarm' device. is constructed to. haveequal sensitivity for the frequencies f1: fir. and the oscillator is caused totransmit these. frequencies. either simultaneously or one by one. Nb signals willlnow normally reach the alarm means L. As a result of a fault in one of the control devices a low attenuation isobtained in the corresponding attenutator and a signal will reach the alarm means causingoperation of lastmentioned means.

Which of the attenuators. that provokes alarm may be determined by either transmitting, from the oscillator O or by receiving in the alarm means I one by one of the frequenciesji in; a

The circuits described above h'avingseries-connected attenuator-s may e. g. be used for giving different alarms for different kinds of faults at the same place, or they may alsobe used for supervisingunattended repeater stations from an. attended station e. g. in case of'a transmission line. In the latter casea special' wire may be reserved for. the fault indication with oscillator, attenuators and alarm-means according to what has been describedabove. Another possibility is to insert the attenuators D1 Du, the oscillator O and the alarm device L into the trans-. mission line itself with its ordinary line amplifiers F according to Fig; 22. V g r In Fig. 22 the oscillator'O and thealarmmea'ns L are connected in shunt tothe transmission line. Of the attenu'ato'rs with their control wires only the ptlr is shown; In this case the frequenci'esfi, f2 he have to be" It is, however, in certain Thismay be an chosen so as. notto; disturb the ordinary transmissionbut they should still be transmitted without too 'largean overs In other respects the oscillator, the

all transmission loss. alarm means and the attenuators D1 Dn may; be arrangedaccording to the'above mentioned principles.

In the circuit according to Fig. 2 2 the attenuators are inserteddirectly into the transmission line in series with the lineamplifiers' E. These latter are always provided' with a strongly negative feedback because of the stability and intermodulation requirements; The attenuators I) may then'instead be inserted in series with the normalfeedbacknetworks of the line amplifiers? in the manner shown in Fig. 2.3. w I

In Fig. 23 a, b anda', 12' indicate the transmissionline wherein the linesamplifier F is: inserted. The. amplifier gets its feedback voltage via the wires c,;d, c, the ordinary feedback-network pi and the attenuator D. In the same manner as-before the controlwire for faults is. connected to the pair of terminals 1 2 of the attenuator D. In this case the attenuator must only comprise positive impedances andshall always have a low attenuation for frequencies differing from its indication frequency f1 If a fault gives the network D a high attenuation, for the frequency f1 instead of a normally low attenuation, a corresponding increase of the amplification ofthe amplifier F at the frequency 1% will be obtained and vice versa,

This fact may then be further utilized according to previous principles for indicating alarm. By inserting the attenuator in the feedback path there is obtained the advantage of being free from the rigorous demands'of impedance correctness at the ordinary transmission frequencies, which demandseach device inserted in a transmission line must fulfil.

In the foregoing description there have only been de= scribed circuits for the indicating equipment which have separate oscillator and indicator; It may also be advantageous .to combine these into a single amplificr-oscilIa torif there is only one? attenuator network. T hiscaseis represented in Fig. 24: V p

The oscillator'OF' in Fig. 2i4'isconnected to-the attenuation network D in such a manner, that the changes of the attenuation of the network D, which-are caused bya fault in a controlled device Will eithe'r start or stop electrij cal'oscillations of the oscillator OF so as to causealarmgiving. The control wirefor the changesof the imped ance caused by faults-is as before connected to the pair of terminals 1--2 of the attenuating ne't'vv'orlcD. The

alarmindicating equipment is in' the figure shown as af relay R. The device OP is' constructed as anamplifier with branches for bothpositive and-negative feedback,

the attenuating network B being inserted in one ofthc'se branches. When the positive feedback is predominating theoscillatbr will'oscillate; otherwise -not.- 7

The attenuatin'g networks- D-may be'const'ructed-ina great many different manners. cases the'impedance' Zin of the control wire constitutes the shunt or series branch of an L.-sec tio'n. Another'con ceivablejpossibility'isto make Zin constitute 'an impedance; of the branch "of" a bridge. In orde'rto make the attenuators frequency selective the control wire rnay' be connectedvia an electrical band pass'filter. In the siinplest case this" filter may consist of" a' single tuned circuit,'- a piezo-electrical' crystal etc.

In Fig. 25 there is shown the principal circuit'of an I oscillator-amplifier device with attenuating networlcs as parts of a" bridge. The electronvalves V1 and V2 are included ina conventional'amplifie'r circuit} Through the output transformer TL the valve"V2 w'orks upon-an alarm circuit L ("a rel'ay) The amplifier'is' g'iven' feedback via the attenuating 'networliD', which here is fo'rnredby'the resistors R1, R2-,'Rs and the transformer T2. The control wire is connected to" the terminals 1-2. The

transformer T2 istuned to a certain frequency bymeansof the condenser C, the transformation ratio of the-trans} former being 111:1. Theamplifie'ris given negative feedbackviathe bridge branch Ri-Rz-and-'positivewia In the less complicated T2. When the value of the impedance, which is connected to the terminals 1-2, exceeds the value the positive feedback is greater than the negative, so that the amplifier will oscillate, if its amplification without feedback is high enough.

The apparatus according to the invention has in the foregoing been described in connection with a plurality of different embodiments. It is, however, obvious that other modifications fall within the scope of the invention.

What is claimed is:

Apparatus for indicating abnormal operation of one or more electrical devices within a group of such devices comprising, in combination, indicating equipment having an oscillator, attenuator means, an alarm, said means be ing connected directly to modify the oscillator output to the alarm, control Wires reflecting by variable impedance changes the condition of supervised apparatus, said attenuator means being connected to said control Wires and having non-linear transmission properties in response to variations in impedance between control wires, a control means connected to the control wires for each device within the group, the impedance of each control means during normal operation being arranged to be responsive continuously to electrical conditions in the corresponding device, the impedance change of each control means being abnormally great, when conditions within its corresponding device exceed limits fixed for maximum deviations during normal operations, said control means being all connected in series to each other and to said control wires, whereby abnormal operation of any device causes a very high impedance of the corresponding control means.

References Cited in the file of this patent UNITED STATES PATENTS 1,461,790 Sultzer July 17, 1923 2,249,323 Mitchell July 15, 1941 2,260,160 Benning et al. Oct. 21, 1941 2,315,434 Leibe Mar. 30, 1943 2,460,789 Thompson Feb. 1, 1949 2,509,365 Parmentier May 30, 1950 2,572,292 Wittenberg Oct. 23, 1951 2,574,458 Atkinson et al. Nov. 13, 1951 2,581,056 Walmsley et al. Jan. 1, 1952 2,636,164 Lubin et al. Apr. 21, 1953 

